1. Field of the Invention
The present invention relates to a secondary battery cell
2. Description of Related Art
A cylindrical secondary battery cell, a typical example of which is a lithium secondary battery cell, includes an electrode group made up with a positive electrode with a positive electrode mixture layer formed thereupon and a negative electrode with a negative electrode mixture layer formed thereupon, wound around a winding core via a separator. The layer of positive electrode mixture is formed on both surfaces of a positive electrode sheet. In the lateral border area close to one side edge extending along the length of the positive electrode sheet, no positive electrode mixture layer is formed, thus a positive electrode mixture untreated portion is formed.
Positive leads normally referred to as tabs, to be used when welding the positive electrode to a positive electrode current collecting member are formed at the positive electrode mixture untreated portion, as integrated parts of the positive electrode sheet through a cutting process or the like. Likewise, the negative electrode mixture layer is formed at both surfaces of a negative electrode sheet on the negative electrode side, and negative leads to be welded onto a negative electrode current collecting member, are formed as integrated parts of the negative electrode sheet at a negative electrode mixture untreated portion located in the lateral border area close to one side edge of the negative electrode sheet extending along the length of the negative electrode sheet, through a cutting process or the like.
The positive electrode and the negative electrode in the electrode group may become short-circuited, since for instance, the burrs or the like, which may result from forming the positive leads or the negative leads through the cutting process, break through the separator. Such spot short-circuiting of the positive electrode and the negative electrode will result in a failure to achieve the required voltage, leading to poorer battery cell performance.
A structure known in the related art addresses this issue by folding back an adjacent separator so as to cover one side edge extending along the length of the positive electrode sheet, where the positive leads are located, and cover the side edge on the opposite side at the positive electrode sheet at which burrs tend to occur readily. This structure is supposed to effectively prevent internal short-circuiting by eliminating the risk of burrs or the like breaking through two separator layers. (See Japanese Laid Open Patent Publication No. 2008-004476)